Electron beams in electron tubes are formed using either thermionic or field emitters as an electron gun with a control grid or anode to form the electrons to the desired shape. The electron gun and anode produce a specified number of electrons at a specified velocity for use in electron tubes such as klystrons, traveling wave tubes, and television picture tubes. With thermionic emitters, electrons are boiled off the cathode and are accelerated toward the anode with an energy depending on the difference in potential between the anode and cathode. The majority of the electrons pass through a hole in the anode and into a drift space. With a potential applied across the drift space, the electron stream is deflected, being focused at a selected point on the screen. With no potential applied across the drift space, the electrons travel in a straight line and strike the screen at a selected or known place.
In modern electron tubes, the electron gun employs an electron emitter, an accelerating anode, and a focusing anode which concentrates the electron beam to enhance electron flow through the hole in the accelerating anode. This improves efficiency and eliminates heat related problems occurring when electrons strike the anode. The number of electrons in the electron beam must be constant and controllable, and the energy of all the electrons must be substantially the same for efficient operation. Small changes in emitter temperature result in changes in electron emission. Similarly, even small changes of anode voltage can affect the current. Therefore, anode potential and emitter temperature must be well regulated to provide constant current for proper operation of a thermionically controlled electron gun or electron beam forming device. A field effect electron gun and control electrode can control electron flow without thermionic emission or thermionic interference. U.S. Pat. No. 3,783,325 issued Jan. 1, 1974 to Joe Shelton discloses a field effect electron gun wherein the number of electrons emitted is a function of the electric field. The electric field which is developed between the emitter and the anode is controlled by the emitter and anode structure.
The usual approach to forming electron beams, using either thermionic or field emitters, is to use a gird to form the electrons to the desired shape. When multiple electron beams are required as for multiple beam cathode ray tubes, several individual emitting sources and control electrodes are required. This approach leads to problems due to electrons being intercepted by the grid resulting in grid heating and excessive grid current.